1. Field of the Invention
The present invention relates to portable wireless telephones and, more particularly, to a portable telephone having a headset allowing hands-free operation of the portable telephone in a time division duplex (TDD) or time division multiple access (TDMA) communication system.
2. Statement of the Problem
While many audio devices such as stereos, tape players and televisions can use headsets, most headsets are not used near any transmitted radio frequency (RF) fields. For example, headsets are sometimes used by telephone operators or receptionists but they are only near a conventional telephone, not an RF transmitter. Police radios, aircraft radios, and similar communication equipment use headsets near RF fields, but operate in a simplex mode (push to talk, release to listen) so that the receiver circuits are only active when the transmitter is inactive. In a home cordless phone or cellular phone, the transmitter and receiver are on continuously, but the transmitter and receiver operate at different frequencies. This is called a frequency division duplex (FDD) system. In an FDD system the RF field is not switched on and off during communication and there is little possibility of audible noise being received by the headset.
Radio telephones using TDD/TDMA are becoming increasingly popular. In TDD/TDMA, the transmitted RF field is turned on and off at some rate, for example one hundred times per second. When the RF field is switched at less than about 20 KHz, the switching is at an audible rate. When a conventional headset is connected to the TDD/TDMA transceiver, the long wire connecting a headset to the phone acts as an antenna injecting stray RF energy into both the phone and headset circuits. The stray RF energy is typically rectified by a semiconductor device in the phone or headset creating a DC voltage that pulses at the TDD/TDMA rate. This in turn creates a highly objectionable audible pulsing which is heard by the headset user and transmitted back to other telephones in the communication system.
In existing headset systems it is common that loudspeakers that would otherwise be used to produce an audible signal are mechanically disconnected from the electrical audio signal when the headset plug is inserted into the jack. In other words, when the headset plug is inserted into the jack, the electrical connection between the audio signal and the loudspeaker is mechanically broken and the electrical audio signal is diverted to the headset. This mechanical breaking effectively disables the loudspeaker but does not provide any logic indication that the headset is present. Because of this, devices that use digital control logic cannot adjust their performance based on the presence of the headset. Particularly in wireless telephone applications it is desirable to change the form and/or volume of the ringing signal when the headset is in use. Most often, performance adjustment can only be achieved by the user pressing one or more buttons on the wireless telephone to adjust features such as volume to a comfortable level for the headset. This is not only inconvenient, but detracts from a major purpose of a headset which is to leave the user's hands free for other work.
Another problem with prior headset systems arises because the audible signal is injected directly into the user's ear, any stray noise can cause discomfort and even ear damage if left uncontrolled. As mentioned before, headsets used in RF environments are particularly susceptible to this stray noise. In wireless telephone applications it is important to conserve battery power by placing the phone in a "standby" or "sleep" mode between calls in which power supplies are poorly regulated and noisy as a result of conserving battery power. This standby power supply noise can be audible and objectionable. A headset for wireless telephones is needed that can reduce stray noise in the headset during standby mode.
A headset is typically provided with a multi-conductor wire coupled to the speaker and microphone elements in the headset. The multi-conductor wire terminates in a plug having the same number of conductors as the wire. The plug is inserted into a jack that is built into a telephone, for example. Alternatively, the jack may be provided in a separate headset interface device. The headset can be given greater functionality by providing more conductors in the jack plug and wire. For example, in a wireless telephone it is desirable that enough conductors exist in the wire, plug and jack to carry signals from the headset microphone, to the headset earpiece and to allow the user to take the phone on and off hook to answer and terminate calls. This functionality comes at great cost and bulk, however, because custom jacks and plugs must be designed if many conductors are used. A headset system that provides a high level of functionality with few conductors in the jack, plug and multi-conductor wire is needed.
3. Solution to the Problem
The above mentioned problems and others are solved by a headset system particularly adapted for wireless telephones operating in a TDD/TDMA environment using filtering circuits in both the wireless telephone and the headset to decrease the quantity of noise injected into the headset and to attenuate any noise that is injected to acceptable levels.
An audio speaker line is coupled through a resistor to a logic level voltage supply to generate a logic level signal indicating that the headset is plugged in. The logic level signal is generated on the audio speaker line, so no additional conductors are required between the wireless telephone and the headset to generate this logic level "headset detect" signal. This headset detect signal is then used by digital control logic to enable or disable the headset and internal speakers to provide features unique to the headset user and to eliminate excessive noise to the headset user.
A microphone current detect circuit is coupled to the microphone audio line in the wireless telephone to provide a logic level "microphone detect" signal indicating when current is flowing in the microphone. The headset is modified by placing a switch in the microphone audio line to stop current flow in the microphone when the switch is open. The control circuitry interprets the microphone detect signal as an on/off hook indicator, thereby allowing the user to answer a call without touching the wireless telephone. Also, by using the microphone audio line for this logic signal, additional functionality is achieved without additional conductors between the wireless telephone and the headset.